The present invention relates to hydraulically operated downhole tools and in particular, though not exclusively, to a control sub to provide selective control of a hydraulically operated expander tool for tubulars.
It is known in the art to utilise the pressure of fluid pumped through a work string in a well bore to control a hydraulically activated tool in the well bore. For instance, when expanding tubulars such as slotted, screen or solid pipe a rotary expander may be used. These expanders have a cone head with an outer diameter greater than the diameter of the tubular. On the tool are arranged hydraulically operated rollers. When mounted on the end of a work string and inserted into a tubular, hydraulic pressure introduced to the expander tool will force the cone through the tubular and with the aid of the rollers the tubular will be expanded to the diameter of the expander tool.
The hydraulic pressure to operate these tools is typically supplied from the surface of the well bore by pumps. Due to the distances of travel to the location of the expander tool it is difficult to control the operation of the expander tool and, in particular, to provide a constant pressure to give a uniform control and therefore expansion of the tubular in the well bore. It is also difficult to start and/or stop the expander tool at desired locations in the well bore.
It has been recognised that being able to control the flow of hydraulic fluid adjacent a hydraulically operated downhole tool would be advantageous. U.S. Pat. No. 5,392,862 describes a drilling mud flow control sub that provides the necessary fluid flow and pressure to activate an expanding remedial tool such as an underreamer, section mill or other cutting tool. The sub consists of a cylindrical sub assembly housing forming a first upstream end and a second downstream end. The housing is threadably connected between a drill string at its first upstream end and a tool at its downstream end. Intermediate the upstream and downstream ends is located a drop ball seat so that insertion of a drop ball will prevent hydraulic fluid flow to the tool. A rupture disc is affixed to a hole formed in the control sub wall normal to the sub axis, above the drop ball seat, so that when obstructed fluid is shunted from the sub.
This flow control sub provides means to terminate fluid flow to the tools hydraulically operating mechanism while allowing fluid circulation through the sub when the tool is deactivated ‘while tripping’ and/or rotating the drill string. However a major disadvantage of this tool is in the single function operation i.e. in turning the hydraulic mechanism off. There is no selective control of the tool. Additionally when hydraulic fluid is applied to the tool through the sub the pressure of this fluid can only be controlled from the surface as with the prior art systems. Further a disadvantage is in the length of time taken for the drop ball to reach the seat and the associated difficulties if the single ball does not locate correctly in the seat.
It is an object of at least one embodiment of the present invention to provide a control sub for use with a hydraulically operated downhole tool which allows the tool to be operated in selective on and off configurations.
It is a further object of at least one embodiment of the present invention to provide a control sub for use with a hydraulically operated downhole tool which allows control of the hydraulic pressure delivered to the tool.
It is a yet further object of at least one embodiment of the present invention to provide a control sub for use with a hydraulically operated downhole tool which allows selective control of fluid circulation when the tool is run in or tripped from the well.
It is a still further object of the present invention to provide a method of controlling hydraulic pressure to a hydraulically operated downhole tool in a well bore.